1. Field of the Invention
The present invention relates to an electrically-driven liquid crystal lens, and more particularly, to an electrically-driven liquid crystal lens, which can improve a viewing angle and reduce crosstalk caused at the center of an electrode with relation to a multi-view stereoscopic display fashion using an increased number of views per a viewing zone, and a stereoscopic display device using the same.
2. Discussion of the Related Art
Nowadays, services for rapid dissemination of information, which will be constructed on the basis of high-speed information communication networks, have been developed from a simple “listening and speaking” service, such as current telephones, to a “watching and listening” multimedia type service on the basis of digital terminals used for high-speed processing of characters, voice and images, and are expected to be ultimately developed to cyberspace real 3-dimensional stereoscopic information communication services enabling virtual reality and stereoscopic viewing.
In general, stereoscopic images representing 3-dimensions are realized based on the principle of stereo-vision via the viewer's eyes. However, since the viewer's eyes are spaced apart from each other by about 65 mm, i.e. have a binocular parallax, the left and right eyes perceive slightly different images due to a positional difference therebetween. Such a difference of images due to the positional difference of both the eyes is called binocular disparity. Also, a 3-dimensional stereoscopic image display device is designed on the basis of binocular disparity, allowing the left eye to watch only an image for the left eye and the right eye to watch only an image for the right eye.
Specifically, the left and right eyes watch different two-dimensional images, respectively. If the two images are transmitted to the brain through the retina, the brain accurately fuses the images, giving the impression of reproducing a real 3-dimensional image. This ability is conventionally called stereography, and a display device utilizing this ability is called a stereoscopic display device.
Meanwhile, stereoscopic display devices can be classified according to components of a 3-dimensional reproduction lens. For example, a lens constructed using a liquid crystal layer is called a liquid crystal lens, which will be driven by an electric field. Hereinafter, this kind of lens is called an electrically-driven liquid crystal lens.
Conventionally, a liquid crystal display device includes two electrodes opposite each other, and a liquid crystal layer formed between the two electrodes. Liquid crystal molecules of the liquid crystal layer are driven by an electric field generated when a voltage is applied to the two electrodes. The liquid crystal molecules have polarization and optical anisotropy properties. Here, the polarization property is that, when a liquid crystal molecule is placed within an electric field, charges in the liquid crystal molecule are gathered to opposite sides of the liquid crystal molecule, whereby a molecular arrangement direction is converted according to an applied electric field. The optical anisotropy property is that, owing to an elongated configuration of liquid crystal molecules and the above-described molecular arrangement direction, the incidence direction of incident light is changed, or the path of light to be emitted or polarization degree is changed according to polarization conditions.
Accordingly, the liquid crystal layer represents a difference of transmissivity by a voltage applied to the two electrodes, and an image can be displayed using the transmissivity difference of pixels.
Recently, there has been developed an electrically-driven liquid crystal lens in which a liquid crystal layer serves as a lens using the above-described properties of liquid crystal molecules.
Specifically, a lens controls the path of incident light according to a given position using a refractive index difference between a lens constituent material and air. If different voltages are applied to different positions of the liquid crystal layer to drive the liquid crystal layer by different electric fields, the incident light into the liquid crystal layer undergoes different phase variations, and as a result, the light crystal layer can control the path of incident light like an actual lens.
Hereinafter, a conventional electrically-driven liquid crystal lens will be described with reference to the accompanying drawings.
FIG. 1 is a sectional view illustrating a conventional electrically-driven liquid crystal lens, and FIG. 2 is a graph illustrating phase variation of incident light depending on position when light passes through the conventional electrically-driven liquid crystal lens.
As shown in FIGS. 1 and 2, the conventional electrically-driven liquid crystal lens includes first and second substrates 10 and 20 arranged opposite each other, and a liquid crystal layer 30 interposed between the first substrate 10 and the second substrate 20.
Here, first electrodes 11 are formed on the first substrate 10 and are spaced apart from one another by a first interval. In these neighboring first electrodes 11, a distance from the center of one of the first electrodes 11 to the center of the next first electrode 11 is called a pitch. Repeating the same pitch for each of the first electrodes 11 forms a pattern.
A second electrode 21 is formed throughout a surface of the second substrate 20 opposite the first substrate 10.
The first and second electrodes 11 and 21 are made of transparent metal. The liquid crystal layer 30 is formed in a space between the first electrode 11 and the second electrode 21. Liquid crystal molecules, constituting the liquid crystal layer 30, have a property of responding to the strength and distribution of an electric field, and thus, have a phase distribution similar to the electrically-driven liquid crystal lens as shown in FIG. 2.
The above-described electrically-driven liquid crystal lens is formed under the condition of applying a high voltage to each first electrode 11 and grounding the second electrode 21. Under these voltage conditions, the vertical electric field is strongest at the center of the first electrode 11, and the strength of the vertical electric field decreases away from the first electrode 11. Thereby, when the liquid crystal molecules, constituting the liquid crystal layer 30, have a positive dielectric constant anisotropy, the liquid crystal molecules are arranged according to the electric field in such a way that they are upright at the center of the first electrode 11 and tilt closer to the horizontal plane with increasing distance from the first electrode 11. As a result, in view of light transmission, an optical path is shortened at the center of the first electrode 11, and is lengthened with increasing distance from the first electrode 11. Representing the length variation of the optical path using a phase plane, the electrically-driven liquid crystal lens shown in FIG. 2 exhibits light transmission effect similar to a parabolic lens having a paraboloidal surface.
The above-described electrically-driven liquid crystal lens can be accomplished by providing electrodes on both substrates, respectively, with liquid crystals interposed therebetween and applying voltages to the electrodes, eliminating the need for a lens having a physically formed paraboloidal (convex) surface.
However, referring to FIG. 2, it can be appreciated that achieving the same phase plane as a parabolid using the electrically-driven liquid crystal lens when a voltage is applied to realize imaging is difficult, and in particular, the phase plane seriously deviates from the profile of the parabolic lens at a region corresponding to the center of the first electrode, i.e. at a lens edge. Deviation of the phase plane from the parabolic lens means that light transmission at the corresponding region is carried out differently from the parabolic lens. This may result in a distorted image upon realization of a 3-dimensional screen. Here, the center of the first electrode corresponds to the lens edge, and thus, the lens profile distortion occurs at the lens edge (i.e. at the first electrode). This profile distortion causes crosstalk, i.e. unintended signals, and such a signal distortion region is called an edge error region. In this case, the greater the pitch, which is a crosswise width of each lens region, i.e. the greater the lens area, the greater the resulting lens profile distortion at a specific region distant from the first electrode, due to reduced electric field effects of the specific region.
Here, the first electrode has a bar shape extending into the drawing. The liquid crystal lens, formed by the vertical electric field between the first electrode and the second electrode, has a parabolic profile in a crosswise direction as shown in FIG. 2. Also, with respect to a longitudinal direction of the first electrode (i.e. in the direction into the drawing), liquid crystals of the liquid crystal layer exhibit the same phase difference under the influence of the same electric field. Thereby, the liquid crystal lens is formed, facing into the drawing, in a cylinder shape having a single paraboloidal cross section.
In addition, the liquid crystals of the liquid crystal layer have an optical path difference of a paraboloidal profile in a crosswise direction, to provide the same function as a parabolic optical lens. However, the liquid crystals are driven by the same phase difference in a lengthwise direction, thereby having the same lengthwise orientation. Therefore, the conventional electrically-driven liquid crystal lens can provide a user (i.e. an observer) with the optimum stereoscopic display visibility only when the user is not in motion. However, when the user moves left or right, the user views different sections of the liquid crystal lens, resulting in inaccurate image recognition or a distorted image.
The above-described conventional electrically-driven liquid crystal lens has the following problems.
Specifically, upon formation of the electrically-driven liquid crystal lens, edges of lens regions have a seriously deviated phase from the profile of a lens having a physically-realized parabolic or convex surface, causing distortion of refractive index upon realization of a 3-dimensional image. This results in crosstalk at the edges of the lens regions, making it impossible to display a normal image.
Further, the liquid crystals of the liquid crystal layer have an optical path difference of a parabolic profile in a crosswise direction to provide the same function as a parabolic optical lens. However, the liquid crystals are driven by the same phase difference in a lengthwise direction, thereby having the same lengthwise orientation. Therefore, the conventional electrically-driven liquid crystal lens can provide a user (i.e. an observer) with the optimum stereoscopic display visibility only when the user is not in motion. However, when the user moves left or right, the user views different sections of the liquid crystal lens, resulting in inaccurate image recognition or a distorted image.